The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained by an imaged area and the water or fountain solution is preferentially retained by the non-imaged areas. When a suitably prepared surface is moistened with water and ink is then applied, the background or non-imaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The ink is then transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon a suitable imaging layer, for example a layer that is sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, either imaged or non-imaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are becoming more common. Examples of such plates are described in U.S. Pat. No. 5,372,915. (Haley et al.). They include an imaging layer comprising a mixture of dissolvable polymers and an infrared radiation-absorbing compound.
It has been recognized that a lithographic printing plate also could be created by ablating an IR absorbing layer. For example, Canadian Patent 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose). Such plates were exposed to focused near IR radiation with a Nd++YAG laser. The absorbing layer converted the infrared energy to heat thus partially loosening, vaporizing or ablating the absorber layer and the overlying silicone rubber. Similar plates are described in Research Disclosure 19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates were developed by wetting with hexane and rubbing. Other publications describing “ablatable” printing plates include U.S. Pat. No. 5,385,092 (Lewis et al.), U.S. Pat. No. 5,339,737 (Lewis et al.), U.S. Pat. No. 5,353,705 (Lewis et al.), U.S. Reissued Pat. No. 35,512 (Nowak et al.), and U.S. Pat. No. 5,378,580 (Leenders).
Thermally switchable polymers have been described for use as imaging materials in printing plates. By “switchable” is meant that the polymer is rendered from hydrophobic to relatively more hydrophilic or, conversely from hydrophilic to relatively more hydrophobic, upon exposure to heat.
U.S. Pat. No. 4,034,183 (Uhlig) describes the use of high-powered lasers to convert hydrophilic surface layers to hydrophobic surfaces. A similar process is described for converting polyamic acids into polyimides in U.S. Pat. No. 4,081,572 (Pacansky). U.S. Pat. No. 4,634,659 (Esumi et al.) describes imagewise irradiating hydrophobic polymer coatings to render exposed regions more hydrophilic in nature. U.S. Pat. No. 4,405,705 (Etoh et al.) and U.S. Pat. No. 4,548,893 (Lee et al.) describe amine-containing polymers for photosensitive materials used in non-thermal processes. Thermal processes using polyamic acids and vinyl polymers with pendant quaternary ammonium groups are described in U.S. Pat. No. 4,693,958 (Schwartz et al.). U.S. Pat. No. 5,512,418 (Ma) describes the use of polymers having heat-sensitive cationic quaternary ammonium groups. However, the materials described in this art require wet processing after imaging.
In addition, EP 0 652 483A1 (Ellis et al.) describes lithographic printing plates imageable using IR lasers, and which do not require wet processing. These plates comprise an imaging layer that becomes more hydrophilic upon imagewise exposure to heat.
U.S. Pat. No. 5,985,514 (Zheng at al.) is directed to processless direct write printing plates that include an imaging layer containing heat sensitive polymers. The polymer coatings are sensitized to infrared radiation by the incorporation of an infrared absorbing material such as an organic dye or a fine dispersion of carbon black. Upon exposure to a high intensity infrared laser, light absorbed by the organic dye or carbon black is converted to heat, thereby promoting a physical change in the polymer (usually a change in hydrophilicity or hydrophobicity). The resulting printing plates can be used on conventional printing presses to provide, for example, negative images. Such printing plates have utility in the evolving “computer-to-plate” printing market.
Other imaging materials comprising heat-sensitive polymers are described, for example, in U.S. Pat. No. 6,190,830. (Leon et al.), U.S. Pat. No. 6,190,831 (Leon et al.), and U.S. Pat. No. 6,096,471 (Van Damme et al.).
Generally, printing plates comprise an imaging layer and an outermost protective layer to provide protection from contamination, fingerprints, and debris resulting from handling and imaging. For example, aqueous-based overcoats that may be partially crosslinked are described in JP Kokai 2002-86949 (Fuji Photo).
As noted from the literature relating to this technology, it is preferable to apply a protective layer to an imaging layer out of a predominantly aqueous solvent system so as to enable removal during imaging or post-imaging processing. For example, U.S. Pat. No. 5,506,090 (Gardner Jr. et al.) describes processless printing plates that have a protective top coat prepared from a film-forming water-soluble or -dispersible polymer that can be removed using a fountain solution.
Protective layers may also be disposed over “ablatable” imaging layers in printing plates as described for example in U.S. Pat. No. 6,397,749 (Kita et al.). Water-soluble or water-swellable polymers are used in protective layers over thermoplastic particle imaging layers described in EP 816 070B1 (Vermeersch et al.) and EP 1 106 347A1 (Kita et al.).
However, when an aqueous formulation is used to apply such layers over imaging layers also applied from aqueous solvent systems, the two formulations are likely to mix at the layer interface. This results in unwanted diffusion of components from one layer to another, or unwanted adhesion of a topcoat to an underlying layer such that the removal of the topcoat (which may be desired in a later step in a process) may not completely occur or may leave behind a contaminated or otherwise damaged surface.
Thus, there is a need in the industry for imaging members prepared from aqueous imaging layer formulations and suitable organic solvent-based protective layer formulations that have discrete layers during coating and drying procedures. It is also desirable that the protective layer be readily removable using an aqueous solvent system during imaging and/or post-imaging development without limiting the type of imaging layer or imaging techniques that can be employed.